Anti-lock control operations take place in all ranges of coefficients of friction, beginning with a very low coefficient of friction (μ<0.1) on ice until high coefficients of friction (μ≈1) on dry asphalt.
To reach a shortest possible stopping distance, it is especially important that when braking on roadways with a high coefficient of friction the anti-lock control systems (ABS control systems) will not take up their control activity before the maximum of the so-called μ-slip curve is reached. Otherwise, the ABS system would not utilize the potential of the tire. It would virtually impede the tire in developing the μ-slip curve, and the adherence abilities of the tire would not be utilized in full extent.
To prevent this occurrence, the control thresholds for the ABS braking operation or the control application thresholds must be rated according to the high coefficient of friction conditions.
Recently, the trade press has assessed the braking performance or the stopping distance on dry asphalt at an increasing rate as the decisive criterion for the quality of the tested ABS system. Therefore, the majority of car makers attribute great importance to these tests.
In some of these tests, the vehicles exhibit very high deceleration values. The same vehicles, as soon as they are equipped with different tires, exhibit a distinctly differing braking performance in comparable tests.
Of course, the difference in the performance can be caused because the conditions of measurements differ from each other, e.g. due to differences in the coefficient of friction of the roadway.
However, the specific tests show that the tires partly differ distinctly in the amount of the coefficient of friction, that means the magnitude of the transmittable μ of the μ-slip curve. This fact explains the significant differences in the braking performance of different tire makes and types of tires under otherwise equal conditions (i.e. identical vehicle, on the same test track, identical climatic conditions, etc.).
The essential thing is to treat different tires in such a fashion that the potential of each individual tire is utilized in the best possible manner. This implies that, on the one hand, the tires with lower transmittable adherence values (μ) are not overbraked (e.g. are excessively subjected to brake slip due to decelerated use of the ABS control) and that, on the other hand, the tires with higher transmittable μ should not be underbraked (i.e. are not subjected to brake slip at all due to a premature use of the ABS control). The objective rather is to design the ABS control in such a fashion that each tire is controlled according to its optimum.
The anti-lock control systems (ABS systems) used in series do not comprise longitudinal acceleration sensors apart from a few exceptions in the field of all-wheel driven vehicles. Therefore, determining a maximum deceleration responsive to the roadway is only possible based on the wheel speed signals. As long as the wheels are not yet braked, it is no particular problem to determine the vehicle deceleration. Things are different, however, in the event of full braking because the wheels are always afflicted by slip. In this case, the vehicle speed and the vehicle deceleration aveh=Δvveh/Δt can be determined only in approximation with the use of the prior-art methods by determining and logically combining the wheel rotational behavior of the individual wheels and selecting defined control phases.
In anti-lock control systems, the deceleration a=Δv/Δt, determined either according to the prior-art method or measured by means of a longitudinal acceleration sensor, represents the input value in a progression term which is taken into account for calculating the control thresholds and control application thresholds of the anti-lock control (ABS control). The deceleration and the so-called negative feedback define the application of the ABS control. The negative feedback represents the wheel deceleration value at which the stable branch of the μ-slip curve has not yet been left, i.e. wheel slip ‘does not yet show’, because the maximum transmittable adherence or coefficient of friction (μ-value) has not yet been reached or, specifically, where the ABS control has not yet commenced if it is designed properly.
The negative feedback is deducted from the currently prevailing wheel deceleration awheel for calculating the control application thresholds. Only when the wheel deceleration exceeds the negative feedback value will this be identified as a locking tendency, and the discrepancy from the negative feedback is detected, integrated and evaluated. The integral represents an essential criterion for the detection of an ABS situation, i.e. an ABS control operation or a locking tendency.
If the value of the negative feedback or of the control threshold is too low, a tire with a relatively high transmittable adherence value (μ) is subjected to the ABS control prematurely, i.e. still in the stable range of the μ-slip curve, and, thus, is virtually hindered to utilize the instantaneously existing tire/road adherence value. The stopping distance becomes longer than would be necessary in view of road conditions and the adherence ability of the tire.
If, however, the value of the negative feedback or of the control threshold is too high, a tire with a relatively low transmittable adherence value (μ) is subjected to ABS control too late, i.e. only far in the unstable range of the μ-slip curve, and, thus, is virtually forced into deep slip up to a wheel lock condition. The result would be an ‘inhomogeneous’ ABS control with excessive pressure modulation. This would cause major losses in comfort and a significant impairment of the braking performance.
Therefore, an object of the invention is to develop a method permitting a still better adaptation of an ABS control system to the different adherence values dependent on the wheel or on the type of tire.